Recently, new magnetic resonance examination system designs have been proposed, in which the main magnet system comprises magnet coils of superconductive material, and in which the magnetic field gradient system is located at the outside of the superconducting coil system and a weak-iron flux conduction system is provided to guide the magnetic gradient flux into the patient bore. A detailed description of such a magnetic resonance examination system is given in U.S. Pat. No. 7,417,434. A main advantage of such new magnetic resonance examination systems is a reduced scanner acoustic noise (“silent imaging”). Furthermore, the superconductive coils of the main magnet are closer to the examination region without compromising the effective bore size. Accordingly, less superconductive material is needed, which reduces the overall costs of the magnetic resonance examination system.
However, the arrangement of the magnetic field gradient system outside of the main magnet of the magnetic resonance examination system influences the main magnet's operation. The weak-iron flux conduction system has a relatively high energy dissipation into the superconducting main magnet coils due to magnetic hysteresis in the flux conduction system. In particular, the timely and spatially changing gradient magnetic fields caused by the magnetic field gradient system leads to dynamic AC losses within the windings of the main magnet coils. The main sources of such AC losses are magnetization losses, eddy current losses, self-field losses, dynamic resistance losses, transport current losses, the resistive loss and the loss due to the flux motion. Such AC losses may lead to a dynamic heat load, which may locally increase the main magnet's temperature. This can initiate a rapid loss of field strength (“quench”) in the superconducting magnet. If the magnet is made of high Tc superconductive material, heat conduction is very slow, which complicates an efficient temperature control of the main magnet considerably.
From conventional magnetic resonance examination systems, in which the magnetic field gradient system is provided inside the main magnet, a large number of heat management solutions are known. The aim of these approaches is to limit the temperature in the examination space, where the patient is situated, i.e. inside the magnet bore. For this purpose temperature sensors are positioned in the magnet bore and gradient coil heating is controlled depending on the sensor data. An example of such a conventional magnetic resonance examination system is given in U.S. Pat. No. 7,209,778. However, since such systems are not exposed to dynamic heat loads, the known solutions are not suitable to ensure stable operation of the superconducting main magnet of a magnetic resonance examination system with a magnetic field gradient system being disposed outside of the main magnet.